Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines The gases in the cylinder and the crankcase are at atmospheric temperature at the commencement of the process. The crankcase pressure is set at particular levels to produce differing values of scavenge ratio, SRV, during each experiment. At the conclusion of a single cycle, the crankshaft is abruptly stopped at tdc by a wrap-spring clutch brake, retaining under the movable cylinder head the trapped charge from the scavenging flow. The movable cylinder head is then released from the top piston rod and depressed so that more than 75% of the trapped gas contents are forced through a paramagnetic oxygen analyzer, giving an accurate measurement from a representative gas sample. If i>o is the measured oxygen concentration (expressed as % by volume) in the trapped charge, Mon is the molecular ratio of nitrogen to oxygen in air, and Cpm is a correction coefficient for the slight paramagnetism exhibited by carbon dioxide, then Sweeney [3.20] shows that the volumetric scavenging efficiency, SEV, is given by: SEV 1- l + Mon -22on 100 1 _ C 1 + M (3.2.2) The correction coefficient for carbon dioxide in the presence of oxygen, Cpm, is a negative number, -0.00265. The value of Mon is traditionally taken as 79/21 or 3.762. The scavenge ratio, SRV, is found by moving the piston, shown as item 5 in Fig. 3.6, inward at the conclusion of the single cycle experiment until the original crankcase state conditions of pressure and temperature are restored. A dial gauge accurately records the piston movement. As the piston area is known, the volume of charge which left the crankcase, Vcc, is readily determined. The volume of charge, Vc, which scavenged the cylinder, is then calculated from the state equation: Pcc v cc .. Pcy V c T T (3.2.3) The temperatures, Tcy and Tcc, are identical and equal to the atmospheric temperature, Tat. The cylinder pressure, pcy, is equal to the atmospheric pressure, pat. The cylinder volume can be set to any level by adjusting the position of the cylinder head on its piston rod. The obvious value at which to set that volume is the cylinder swept volume, Vsv- The scavenge ratio, SRV, is then: V SRV = —— (3.2.4) V SV In the final paragraph of Sec. 3.1, the desirability was emphasized of conducting a relevant scavenging experiment in an isothermal, isovolumic and isobaric fashion. This experiment satisfies these criteria as closely as any experiment ever will. It also satisfies all of the 226
Chapter 3 - Scavenging the Two-Stroke Engine relevant criteria for dynamic similarity. It is a dynamic experiment, for the piston moves and provides the realistic attachment behavior of scavenge flow as it would occur in the actual process in the firing engine. Sweeney [3.20, 3.23] demonstrates the repeatability of the test method and of its excellent correlation with experiments conducted under firing conditions. Some of those results are worth repeating here, for that point cannot be emphasized too strongly. The experimental performance characteristics, conducted at full throttle for a series of modified Yamaha DT 250 engine cylinders, are shown in Fig. 3.7. Each cylinder has identical engine geometry so that the only modifications made were to the directioning of the transfer ports and the shape of the transfer duct. Neither port timings nor port areas were affected so that each cylinder had almost identical SR characteristics at any given rotational speed. Thus, the only factor influencing engine performance was the scavenge process. That this is significant is clearly evident from that figure, as the bmep and bsfc behavior is affected by as much as 15%. When these same cylinders were tested on the single-cycle gas scavenging rig, the SEV-SRV characteristics were found to be as shown in Fig. 3.8. The figure needs closer examination, so a magnified region centered on a SRV value of 0.9 is shown in Fig. 3.9. Here, it can be observed that the ranking order of those same cylinders is exactly as in Fig. 3.7, and so too is their relative positions. In other words, Cylinders 14 and 15 are the best and almost as effective as each other, so too are cylinders 9 and 7 but at a lower level of scavenging efficiency and power. The worst cylinder of the group is cylinder 12 on all counts. The "double indemnity" nature of a loss of 8% SEV at a SRV level of 0.9, or a TEV drop of 9%, is translated into the bmep and bsfc shifts already detailed above at 15%. A sustained research and development effort has taken place at QUB in the experimental and theoretical aspects of scavenging flow. For the serious student of the subject, the papers published from QUB form a series, each using information and thought processes from the preceding publication. That reading list, in consecutive order, is [3.6], [1.23], [3.13], [3.20], [1.10], [3.23], [1.11], and [3.17]. 3.2.4 Comparison of loop, cross and uniflow scavenging The QUB single-cycle gas scavenge experiment permits the accurate and relevant comparison of SEV-SRV and TEV-SRV characteristics of different types of scavenging. From some of those previous papers, and from other experimental work at QUB hitherto unpublished, test results for uniflow-, loop- and cross-scavenged engine cylinders are presented to illustrate the several points being made. At this stage the most important issue is the use of the experimental apparatus to compare the various methods of scavenging, in order to derive some fundamental understanding of the effectiveness of the scavenging process conducted by these several methodologies. In Sec. 3.3 the information gained will be used to determine the theoretical relevance of this experimental data in the formulation of a model of scavenging to be incorporated within a complete theoretical model of the firing engine. Figs. 3.10-3.13 give the scavenging and trapping characteristics for eight engine cylinders, as measured on the single-cycle gas scavenging rig. It will be observed that the test results fall between the perfect displacement line and perfect mixing line from the theories of Hopkinson [3.1]. By contrast, as will be discussed in Sec. 3.3, some of the data presented by others [3.3, 3.4] lie below the perfect mixing line. 227
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Design and Simulation of Two-Stroke
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A Second Mulled Toast When as a stu
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Acknowledgments As explained in the
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Table of Contents Nomenclature xv C
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Table of Contents 2.10.1 Flow at pi
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Table of Contents 3.5.3.1 The use o
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Table of Contents 6.1.3 The effect
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Nomenclature NAME SYMBOL UNIT (SI)
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speed of rotation speed of rotation
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attenuation or transmission loss wa
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Chapter 4 Combustion in Two-Stroke
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Chapter 4 - Combustion in Two-Strok
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a stratilpl o tions to ivior. If m
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CD 30 -, W 20 - LU CO iS _J LU OC 1
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181 also x3 = — = 0.905 x6 = 2.17
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to be cur .3.20) .3.21) .3.22) for
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3.23) com- :on is hhas has a /eng-
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stroke .3.29) mean me to 3land ssio
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a LU z: en 3 CO a LU z EC D m O ho
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CD 1.2 -i 1.0 - di „„ LU 0.8 cc
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CO —> 111" cc LU < LU _J LU CC £
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Combustion in compression-ignition
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stroke ari more often lseful lental
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lat the ssi ;tribu- ELEVATION VIEWS
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CYLINDER HEAD TYPE IS CENTRAL BORE,
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CO E 50 40 - O O _i Hi > I CO 30 Z)
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CURRENT INPUT DATA FOR 2-STROKE 'SP
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Cylin- PP £ Pa- tics of Paper Chap
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vfitro- ;titi >tants 1970. i(In- Ch
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£2 = Pi P2 I Pi J Chapter 4 - Comb
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CHAINSAW AT 9600 rpm. Chapter 4- Co
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LLI •z. O N -z. 0.21 -, 0.20 DC 0
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Chapter 7 Reduction of Fuel Consump
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Chapter 7 • Reduction of Fuel Con
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Chapter 7 - Reduction of Fuel Consu
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^ s C£ Q 2^ E Q. Q_ z' o ISSI HCEM
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0.35 • 0.34 LU ° 0.33 - c 03 DC
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§ 2000
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Chapter 7- Reduction of Fuel Consum
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Chapter 8 Reduction of Noise Emissi
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Chapter 8 • Reduction of Noise Em
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Chapter 8 - Reduction of Noise Emis
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• • / . • Appendix Listing of
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Active radical (AR) combustion and
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initial conditions, assumptions reg
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in two-stroke engine (schematic dia
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exhaust timing control valve, effec
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I Exhaust emissions/exhaust gas ana
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Exhaust systems (continued) untuned
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gas constant/universal gas constant
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in crankcase heat transfer analysis
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Mercury Marine air-assisted fuel in
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I PISTON POSITION (computer program
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Pressure wave reflection (in pipes)
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local acoustic velocity, 60 local M
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Roots blower in turbocharged/superc
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Scavenging (continued) scavenging e
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Simulation, engine See Computer mod
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temperature vs. crankshaft angle (c
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work per thermodynamic (Otto) cycle
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Design and Simulation of Two Stroke Engines

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